These known esters are used industrially as intermediate products, especially for the manufacture of tin derivatives which are useful as heat stabilizers for polyvinyl chloride and, in the case of the lower alcohol esters (especially R=methyl), for the manufacture of heterocyclic (thiophene or thiazole) derivatives which are useful as plant-protection agents.
The main route for obtaining esters (I) is the esterification of an alcohol ROH with an acid HS--X--COOH, the latter being obtained by the action of a hydrosulphide on the corresponding chloro- or bromocarboxylic acid. Despite its excellent overall yield, this method has the disadvantage of generating a considerable quantity of aqueous saline waste.
It is known that esters of mercaptocarboxylic acids can also be prepared by the action of an alkali metal hydrosulphide on an ester of a halocarboxylic acid. This method has been employed especially by R. M. Acheson et al. (J. Chem. Soc. 1961, pp. 650-660, in particular p. 656) for preparing methyl .alpha.-mercapto-.alpha.-methylpropionate by the action of sodium hydrosulphide on methyl .alpha.-bromo-.alpha.-methylpropionate in anhydrous methanol; the yield is very low (approximately 36%). Additional systems are described in Patent Applications JP 48-86818, 63-10755 and 2-304061, both of which operate in the presence of water, either in a hydroalcoholic medium (JP 48-86818 and 63-10755) or in a water-toluene medium in the presence of a phase transfer agent (JP 2-304061).
It has now been found that the presence, in the reaction medium, of appreciable quantities of water (other than that possibly originating from the in-situ formation of the hydrosulphide) is unfavorable for the selectivity of the reaction and that this selectivity is moreover improved by operating at a high pressure of hydrogen sulphide.